This invention relates to computer systems and circuits; and more particularly, to methods and apparatus for generating an appropriate reference voltage level for differential signaling.
Computer systems are consistently being updated to operate at faster rates. In doing so, many of the components of these system must operate within new parameters, or be replaced at an additional cost. Achieving these higher operating rates further requires devices within these systems to communicate among themselves at faster rates.
A technique employed to increase the rate at which two devices can communicate with each other is decreasing the voltage swing between a communication signals high and low values. Typically, this includes reducing the supply voltage level at which these devices operate. This decrease in voltage swing allows a signal to transition faster between values. However, as this voltage swing is decreased, additional precautions must be taken to ensure that noise does not interfere with the signal.
Differential signaling is one method used in communicating between devices in a system and is useful in avoiding errors induced by noise. Typically, each sending device provides a voltage reference and a communication signal which are typically connected to a differential amplifier. Because approximately the same noise is typically induced on both the voltage reference and a communication signal, a receiving differential amplifier produces a relatively xe2x80x9ccleanxe2x80x9d communications signal.
However, not all sending devices can provide an appropriate reference voltage. For example, when a system is upgraded or a new version is introduced that requires a different reference voltage level and possibly uses a different supply voltage, an older component of the system might not be able to provide the new reference voltage.
Such is the problem introduced by the Accelerated Graphics Port (xe2x80x9cAGPxe2x80x9d) Interface Specification Revision 2.0, May 1998, which requires a universal AGP target (such as the 82465GX GXB manufactured by Intel Corporation) to support both 3.3 Volt and 1.5 Volt operation on the AGP interface supply voltage (Vddq). The specification stipulates a voltage reference that is nominally 0.5*Vddq for 1.5 Volt operation, and nominally 0.4*Vddq for 3.3 Volt operation. Therefore, the value of the reference voltage must be adjusted to match the requirements based on the supply voltage used. What is needed is a system for efficiently providing the appropriate reference voltage.
According to the invention, systems, apparatus and methods are disclosed for providing one or more bimodal reference voltages for using in differential signaling. Typically, a system comprises a first component requiring a first reference voltage; a second component requiring a second reference voltage; a power source to produce a supply voltage; a first voltage reducer electrically coupled to the power source, the first voltage reducer to produce a third reference voltage using the supply voltage; a multiplexor electrically coupled to the first voltage reducer to produce the first reference voltage by selecting between at least the third reference voltage and a fourth reference voltage; and a second voltage reducer electrically coupled to the power source and to the second component, the second voltage reducer to produce the second reference voltage using the supply voltage.